There is no single generator wattage for every house. An essentials-only backup may need a fraction of the power required by an all-electric home running HVAC, water heating, cooking, laundry and EV charging at the same time.
Size from actual running loads plus the largest motor-starting demand, then decide which loads can be managed or left off during an outage. Do not use square footage or the main-breaker rating as the answer.
Start with the loads you need
Essentials often include refrigeration, lighting, internet equipment, a gas-furnace blower, a sump or well pump and medical devices.
Whole-home comfort can add central air conditioning, heat pumps, electric water heating, cooking, clothes drying, pool equipment and EV charging. These high-demand loads determine whether a modest generator is enough.
How to calculate generator watts
- List every load that may operate at the same time.
- Record running watts from the equipment nameplate or manufacturer.
- Identify the largest additional starting watts for a motor or compressor.
- Add total running watts plus that starting requirement.
- Check 120-volt and 240-volt needs, outlet limits and load balance.
- Add sensible operating margin and use the generator’s fuel-specific continuous rating.
For equipment labeled only in volts and amps, a simple resistive-load estimate is watts = volts × amps. Motors, power factor and nonlinear electronics may require a more detailed calculation.
Starting watts change the answer
Refrigerators, pumps, furnace blowers and air conditioners can draw much more current during startup than while running. If two large motors start together, the generator may bog down or trip even when steady load looks acceptable.
Soft starters and load-management controls can reduce simultaneous demand when they are approved for the equipment and correctly installed.
Selected circuits versus whole home
A portable generator through approved transfer equipment usually backs up selected circuits. A standby generator can support selected circuits or the whole panel, but “whole home” does not mean every appliance can operate without coordination.
Load management can temporarily block water heaters, EV chargers, pool pumps or secondary air conditioners so essential circuits stay within capacity.
Final sizing checks
- Propane versus natural-gas output
- Fuel meter, regulator, tank and pipe capacity
- Altitude and temperature derating
- Transfer switch and service configuration
- Neutral and grounding requirements
- Voltage dip and frequency during motor starts
- Future loads and expected outage duration
Have a qualified installer complete the electrical load calculation and fuel assessment before buying a home standby system.
Worked sizing example
Suppose the outage plan includes a refrigerator at 700 running watts with 2,200 additional starting watts, a furnace fan at 800 running watts with 2,350 additional starting watts, a sump pump at 1,050 running watts with 2,200 additional starting watts, plus 500 watts for lights, internet equipment and essential electronics.
The running total is 3,050 watts. Add the largest single starting requirement, 2,350 watts, for a calculated minimum of 5,400 watts. A generator must exceed that demand at the fuel and site conditions in which it will operate, and its transfer equipment and individual outlet must also carry the load. This Generac worksheet example is a planning method, not a substitute for the actual appliance labels or an installer load calculation.
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